BSE calculations in spin-polarization systems

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yuanchang_li
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BSE calculations in spin-polarization systems

#1 Post by yuanchang_li » Wed May 19, 2021 7:10 am

Dear all,
When I do a spin-polarization BSE calculation by using VASP, is it possible to calculate the transition between different spin channels?
When I check my BSEFATBAND file, it seems that I can only obtain the transition between the same spin channel, probably because of the spin selection rule.
Thanks a lot.

kresse_georg
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Re: BSE calculations in spin-polarization systems

#2 Post by kresse_georg » Mon Aug 02, 2021 12:01 pm

Dear all,

When the BSE equation is solved, one makes an ansatz for the excited state. In VASP, and all BSE codes that I know, this ansatz involves a hole and an electron pair prepared within each spin channel (for instance an electron with up spin is removed from the ground state determinant and placed with the same spin into a previously unoccupied orbital). If the ground state is not spin polarized, the excited state is hence corresponding to a singlet.

Since the Hamiltonian commutes with the spin operator, this ansatz is- as far as I recall -exact. In order to obtain excitations between different spin channels, your Hamiltonian must involve terms that do not commute with the spin operator. This means that you have to include spin-orbit coupling (LSORBIT = .TRUE.) and perform BSE calculations also using LSORBIT = .TRUE. to obtain excitations between the different spin channels. However, the transition probabilities between different spin channels will be small (selection rules still apply to some extent).

Alternatively, if your groundstate is not spinpolarized (ISPIN=1), you can specify LHARTREE=.FALSE or the undocumented flag LTRIPLET = .TRUE. in the INCAR file (for the BSE calculation). This assumes that you remove an up electron from the ground state determinant and place a down electron in a previously unoccupied orbital (so the resulting ansatz corresponds to a triplet state). This will yield many transitions with zero transition probabilities. However, I am not sure whether the BSEFATBAND approach "interprets" this case correctly. Also, I do not recall whether we have tested this approach for spin polarized ground states i.e. ISPIN=2 (I believe that I tested this only for anti-ferromagnetic systems).

Georg

marie-therese.huebsch
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Re: BSE calculations in spin-polarization systems

#3 Post by marie-therese.huebsch » Tue Aug 03, 2021 12:26 am

The following is a related post: LTRIPLET in BSE calculation!

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